Chromatography is the collective term for a set of laboratory techniques for the separation of mixtures. The mixture is dissolved in a mobile phase, which carries it through a stationary phase. The various constituents of the mixture travel at different speeds, causing them to separate. The separation is based on differential partitioning between the mobile and stationary phases. Subtle differences in a compound's partition coefficient result in differential retention on the stationary phase, thus changing the separation. Chromatography is useful for the separation of compounds that are structurally related, such as regio-isomers, chiral, diastereomers, etc. Some techniques, including SFC, are known for being particularly useful for separating structurally related vitamins, natural products and chemical materials. Often, however, chromatographic techniques are insufficient to separate all structurally related compounds. For example, critical pairs of related vitamins (e.g., D2 and D3, K1 and K2) are difficult to separate/resolve.
Packing materials for fluid or liquid chromatography can be generally classified into two types: organic materials (e.g., polydivinylbenzene) and inorganic materials (e.g., silica). Many organic materials are chemically stable against strongly alkaline and strongly acidic mobile phases, allowing flexibility in the choice of mobile phase composition and pH. However, organic chromatographic materials can result in columns with low efficiency, particularly with low molecular-weight analytes. Many organic chromatographic materials not only lack the mechanical strength of typical chromatographic silica and also shrink and swell when the composition of the mobile phase is changed.
Silica is widely used in High Performance Liquid Chromatography (HPLC), Ultra High Performance Liquid Chromatography (UHPLC), and Supercritical Fluid Chromatography (SFC). Some applications employ silica that has been surface-derivatized with an organic functional group such as octadecyl (C18), octyl (C8), phenyl, amino, cyano, and the like. As stationary phases for HPLC, these packing materials can result in columns that have high efficiency and do not show evidence of shrinking or swelling.
Hybrid materials can provide solutions to certain chromatographic problems experienced with silica based packing materials. Hybrid materials can provide improvements including improved high and low pH stability, mechanical stability, peak shape when used at pH 7, efficiency, retentivity, and desirable chromatographic selectivity.
However, potential problems can exist for conventional hybrid materials and silica materials in other applications. One problem is poor peak shape for bases when used at low pH, which can negatively impact loadability and peak capacity when used at low pH. Another problem is a change in acidic and basic analyte retention times (denoted ‘drift’) after a column is exposed to repeated changes in mobile phase pH (e.g., switching repeatedly from pH 10 to 3).
Another problem is retention drift or change, for example in chromatography modes with little water (e.g., less than 5%, less than 1%). For example, retention drift or change is observed under standard SFC conditions for both silica and organic-inorganic hybrid (e.g., BEH Technology™ materials available from Waters Technologies Corporation, Milford Mass.) based chromatographic phases, bonded and unbonded. Other SFC stationary phases can also exhibit similar retention drift or change.